Thousands and thousands of children around the world suffer from sickle cell disease and thalassemia major. There is a myriad of other less common serious non-malignant hematological diseases, which have even more devastating effects, for which HSCT remains the only viable cure. Low-dose TBI based conditioning represents a minimally toxic approach to transplantation for these children-a way to overcome alloimmunization, however, is needed to make this approach more effective. Alefacept, the only currently FDA approved agent that specifically targets memory T cells, the investigators believe, holds the key to making low-dose TBI based conditioning more effective and could, thereby, dramatically alter the field of transplantation for non-malignant diseases. sustained donor engraftment needs to be developed.